GEOCHEMICAL AND MINERALOGICAL REACTIONS BETWEEN BIO-OIL, ROCKS, AND SALINE/BRACKISH GROUNDWATER: IMPLICATIONS FOR SUBSURFACE CARBON STORAGE AND CRITICAL METAL EXTRACTION

Bio-oil from pyrolysis of biomass is a promising medium for durable subsurface carbon storage, as well as other industrial and agricultural uses. However, in contrast to supercritical CO₂, little is known (or at least published) about how it reacts with rocks or brackish/saline waters in the subsurface. Bio-oil’s distinctive properties including low pH (2-3), high density (~1.2 g/cm3), and diverse organic compounds have the potential to react with host rocks and native fluids in ways that could affect subsurface properties including porosity/permeability, element mobility, and microbial communities. These properties may also be useful for hydrometallurgical applications.

We performed reaction and leaching experiments between bio-oil, natural brines, and common lithologies (sandstones, limestone, basalt, granite) and critical metal feedstocks (mine tailings, fly ash, ionic clay, peralkaline volcanics, granitic regolith, natural clinker ash) to explore mineralogic changes and elemental leaching, and compared results with leaching by conventional lixiviants sulfuric acid and ammonium sulfate. Bio-oil effectively dissolves calcite, increasing porosity and extracting large fractions of Ca (40-60%, for ~1:1 rock-bio-oil mixes) from calcite-bearing samples. Interestingly, bio-oil also removes large fractions of Mn, but not Fe, from most samples regardless of mineralogy, including 30-60% of Mn from sandstones, granite, basalt, and mine tailings. Bio-oil also extracts significant fractions of heavy metals from some samples, including up to 15-30% of V, 15-55% of Co and Ni, and ~10-60% of U. Bio-oil extracts >20% of REEs from regolith and ionic clay, but <10% from most samples; in most cases with strong preference for M/HREE. Initial results of brine-bio-oil experiments suggest separation of bio-oil’s aqueous component into brine, leaving behind a gel-like organic residue.

Although conditions in natural injection settings no doubt differ from those of our experiments, our results suggest that bio-oil’s strong ability to dissolve calcite and leach significant amounts of Mn and, to a lesser degree, other metals, may be important to consider for managing surface bio-oil carbon storage. More complex reactions between bio-oil, native brackish/saline waters, and host rocks may also produce hybrid aqueous phases with distinctive compositions and properties.